Ultrasound-based measuring systems are used to measure a distance from an object located ahead of a sensor. The used ultrasonic sensors are generally based on the pulse-echo method. During this operation, the sensor transmits an ultrasonic pulse and measures the reflection of the ultrasonic pulse (echo) caused by an object. The distance between the sensor and the object is computed via the measured echo transmission time and the speed of sound.
To measure the distance, the analog echo signal is evaluated in such a way that a valid echo is present exactly at the time when the measured voltage of the echo pulse exceeds a threshold value for an established minimum time period.
The ultrasonic sensors are used here as a transceiver unit. The sensors include a resonant transducer element whose emission frequency is coordinated with the resonance frequency of the transducer.
Due to the resonant character of the transducer, transient oscillations and decays result during the transmission excitation. In particular, the decay processes result in a dead time during which it is not possible to detect the echoes. This dead time is equivalent to a minimum range for the sensor which can presently not be fallen below.
It would be desirable to reduce the minimum range.
It is also known how the dead time may be reduced by a frequency-modulated excitation if, instead of excitation using the resonance frequency of the transducer, a special excitation outside of the resonance, in particular at the end of the excitation, takes place in a targeted manner. Such an excitation of the transducer using a constant frequency, which is different from the resonance frequency, in particular at the end of the excitation, is known from German Patent No. 101 03 936, for example. In this way, the initial amplitudes for the decay may be reduced and thus also the decay time. The disadvantage here is that such ultrasonic measuring systems are designed to generate very special forms of transmission and are thus limited to these very special forms of transmission.